1. Field of the Invention
This invention relates to a method and an apparatus for sensor-less control of a permanent magnet synchronous motor.
2. Description of the Related Art
It is known to control a permanent magnet synchronous motor without using any positional sensor. In one example of known sensor-less speed control, the rotor position and the speed of a permanent magnet synchronous motor are calculated from the instantaneous values of phase voltages and phase currents according to a given voltage equation corresponding to an equivalent circuit of the motor. Drive of the motor is adjusted in response to the calculated rotor position and the calculated speed.
Since the polarity of a magnetic field can not be determined by the above-indicated voltage equation, two different angular positions are present as the calculated rotor position. At start of the motor, the polarity of a magnetic field is examined, and an initial angular rotor position is determined on the basis of the examination result. Then, selection is given of one of two angular rotor positions which has continuity with the initial angular rotor position. Subsequently, selection is periodically given of one of two angular rotor positions which has continuity with the immediatelypreceding selected angular rotor position.
During low-speed drive of the motor, voltage command values are relatively small, and the difference between the voltage command values and phase voltages actually applied to the motor tends to be relatively great due to a dead time. Such a great difference causes a large error in the calculated angular rotor position. Accordingly, there is a chance that the polarity of a magnetic field will be erroneously recognized. In the erroneous recognition, the calculated angular rotor position differs from the actual angular rotor position by an error corresponding to about T radians.
In a typical example of known sensor-less speed control of a permanent magnet synchronous motor, an inverter generates phase currents in response to the output signal of a PWM (pulse width modulation) control circuit. The inverter feeds the generated phase currents to the motor. The phase currents and related phase voltages are monitored. A calculator estimates the rotor position in the motor from the monitored phase currents and the monitored phase voltages. The PWM control circuit is directly or indirectly informed of the estimated rotor position. The output signal of the PWM control circuit depends on the estimated rotor position. Accordingly, the phase currents fed to the motor are adjusted in response to the estimated rotor position.